1. Field of the Invention
The subject invention generally relates to a machinable austempered cast iron article having improved machinability, fatigue performance, and resistance to environmental cracking, a method of making the machinable austempered cast iron article, and a machinable austempered cast iron composition. More specifically, the subject invention relates to a machinable austempered cast iron article having a microstructure of a continuous matrix of equiaxed ferrite with islands of austenite that exhibits improved strength, ductility, machinability, fatigue performance, and resistance to environmental cracking.
2. Description of the Related Art
Regular ductile iron (RDI) articles and regular austempered ductile iron (ADI) articles are well known in the art, as are methods for making these articles. RDI articles are used extensively in automotive applications and ADI articles are used in limited vehicular applications, including crankshaft and chassis components. RDI articles are generally made by casting a ductile iron composition without subjecting the ductile iron composition to a post-casting heat treatment process. The ductile iron composition can vary in percentage of components, but must include carbon and sufficient alloying elements to form a microstructure of well-formed graphite nodules in a matrix of ferrite and pearlite in the regular ductile iron articles.
Typical RDI articles that require a good combination of strength and toughness have the ferritic and pearlitic microstructure, not a substantially pearlitic microstructure. The ferritic and pearlitic microstructure has superior physical properties to the substantially pearlitic microstructure when the ductile iron composition is cast without subjecting the ductile iron composition to post-casting heat treatment. Alternatively, RDI articles can be heat treated by normalizing or quenching and tempering. However, the typical ductile iron compositions do not respond to a heat treatment process of the present invention, forming unwanted pearlite during rapid cooling. Thus, typical ductile iron compositions are not suitable for use with the heat treatment process of the subject invention.
ADI articles are made by subjecting the ductile iron composition to a post-casting heat treatment process. A microstructure of the ductile iron composition prior to the heat treatment process is not a factor and is overlooked, with emphasis on the heat treatment process itself for producing the ADI article. ADI articles are generally produced by austenitizing followed by austempering.
Another method for producing ADI articles is step austenitizing, which is disclosed in “Improving The Properties of Austempered Ductile Iron” to Gundlach (the DIS publication), but remains an experimental method that has not been applied to and optimized for production. Step austenitizing is a process by which the ductile iron composition is heated to and held at an initial austenitizing temperature. The step austenitizing proceeds by quenching the ductile iron composition to sequentially lower temperatures and holding the ductile iron composition at each temperature for a short amount of time. The process ends by quenching the ductile iron composition to produce the ADI article. The ADI article produced through step austenitizing typically has an ausferritic microstructure. The ausferritic microstructure generally provides higher strength than the regular ductile iron articles, but is also less ductile and less machinable than the regular ductile iron articles.
Austenitizing followed by austempering is performed by first austenitizing a ferritic and pearlitic microstructure at an austenitizing temperature, typically in a range of from 1550° F. to 1650° F., although austenitizing temperatures as low as 1450° F., which may be in an intercritical temperature range, have been documented. The ductile iron composition is then austempered at a significantly lower temperature, typically between 350° F. and 725° F., to produce the regular austempered ductile iron article. Austenitizing and austempering temperatures are varied to achieve desired physical properties in the regular austempered ductile iron articles. The resulting regular austempered ductile iron articles have an ausferrite microstructure, i.e., acicular ferrite plus austenite. Time at temperature for the austenitizing and austempering process is also crucial. For articles with the ferritic and pearlitic starting microstructure, the carbon must diffuse into the austenitic matrix from graphite nodules interdispersed throughout the ductile iron composition to form a high carbon austenite before quenching to the austempering temperature. As a result, an austenitizing time of 90 minutes is typical to achieve production of the high carbon austenite.
ADI articles that are austenitized at lower temperatures exhibit better machinability than ADI articles austenitized at higher temperatures. However, the acicular ferritic plus austenitic microstructure (ausferrite) that is produced through austenitizing at the lower temperatures does not have sufficient strength for many applications in which ADI articles are used.
Spanish Patent No. ES 8104423 to Muhlberger discloses a method for producing another austempered ductile iron article having a microstructure of austenite mixed with bainite and spherical graphite. The Muhlberger austempered ductile iron (ADI) article is produced by heat treating a ductile iron composition as shown in Table 1.
TABLE 1ElementWt. %Carbon2.5-3.7Silicon2.0-3.0Manganese  >0-<0.3Copper0.1-1.5Molybdenum0.2-0.8Nickel  0-3.0IronRemainder
The heat treating is performed by austenitizing the ductile iron composition in a temperature range of from 1472° F. to 1580° F. for between 10 and 60 minutes. The ductile iron composition is then quenched over a period of less than 2 minutes to a temperature range of between 662° F. and 752° F. The ductile iron composition is maintained within the temperature range of between 662° F. and 752° F. for a period of between 5 and 60 minutes to produce the Muhlberger ADI article having a microstructure of austenite mixed with bainite and spherical graphite, which is a regular austempered ductile iron structure. The Muhlberger ADI article is insufficient for applications of the subject invention. The molybdenum composition is too high, resulting in the iron article having a Brinell Hardness that is too high, and the composition requires manganese. Furthermore, the resulting microstructure of the austempered ductile iron composition is austenite mixed with bainite and spherical graphite, and does not contain equiaxed ferrite with islands of austenite because the method does not begin with a substantially pearlitic microstructure prior to austenitizing. In addition, the combination of chemistry and austenitizing temperature are not suitable for the subject invention. Referring to FIG. 5, Muhlberger ADI exhibits a different relationship between yield strength and hardness. Thus, the Muhlberger ADI has physical properties that are insufficient for applications of the subject invention.
RDI articles and ADI articles have physical properties that are suitable for many applications, however, RDI articles and ADI articles are often not suitable for the same applications. Referring to FIG. 1, RDI articles can have higher ductility, measured by elongation, than ADI articles. However, for the same strength level, ADI articles have higher ductility than RDI articles. Properties of normalized ductile iron (normalized DI) articles and quenched and tempered ductile iron (quenched and tempered DI) articles are also shown. RDI articles, normalized DI articles, and quenched and tempered DI articles are most commonly used in applications that require extensive machining. Even though physical properties of the articles can be manipulated by adjusting production processes and chemistries of the ductile iron composition, RDI articles, normalized DI articles, and quenched and tempered DI articles do not have sufficient ultimate tensile or yield strength to satisfy strength requirements of many applications.
On the other hand, ADI articles, as shown in FIG. 5, have sufficient strength for many applications that cannot use RDI articles because of lack of sufficient strength. However, ADI articles are significantly less machinable than RDI articles. ADI articles also exhibit insufficient flaw tolerance and insufficient resistance to environmental cracking, i.e., resistance to cracking while being subjected to a combination of strain and various types of fluid such as water, oil, and fuel. As a result, ADI articles show insufficient performance in fatigue life tests, making the ADI articles unsuitable for applications that will subject the articles to cyclical loading and unloading. Furthermore, prior art ADI articles achieve a lowest Brinell hardness (BHN) of 268 BHN. Therefore, the prior art ADI articles are also unsuitable for applications that require extensive machining.
Thus, there remains an opportunity for a machinable austempered cast iron (MADI) article and a method of producing the MADI article having a unique combination of improved strength, ductility, machinability, fatigue performance, and resistance to environmental cracking that has not been achieved by the prior art.